1. Field of the Invention
The present invention relates generally to a fluorescent lamp, and more particularly to a low-wattage fluorescent lamp utilizing only argon as the inert fill gas.
2. Description of Related Art
Standard T8 lamps utilizing only argon as the inert fill gas have a low lumen efficacy, expressed as lumens per watt of positive column power. These same low-wattage T8 lamps yield reduced positive column power through addition of krypton to the fill gas. The addition of krypton reduces energy consumption in fluorescent lamps because krypton, having a higher atomic weight than argon, results in a lower wattage gradient in the positive column with lower heat conduction losses per unit length of discharge in the lamp. However, a major disadvantage of krypton is that it suppresses the Penning ionization effect, thereby making the lamp more difficult to start on standard ballasts. Consequently, an attached starting aid, such as a conductive stripe applied along the length of the lamp, must be used to effect reliable starting of low-wattage fluorescent lamps utilizing krypton in the fill gas. However, the addition of a conductive stripe contributes an additional manufacturing step as well as additional material and manufacturing cost.
Still a further disadvantage of krypton is that striations are substantially more prevalent in fluorescent lamps utilizing krypton in the fill gas than in lamps containing only argon. Striations are an undesirable feature to consumers in a finished fluorescent lamp product.
Still a further disadvantage of krypton-containing lamps is that they are primarily compatible only with instant-start ballasts. Existing low-wattage krypton-containing lamps may not be compatible with rapid-start ballasts.
It is desirable to produce a low-wattage fluorescent lamp containing only argon as the inert fill gas that consumes less energy to achieve similar lumen efficacy as compared to krypton-containing T8 lamps. Such a low-wattage lamp would also substantially eliminate striations associated with krypton-containing lamps. Preferably, such a lamp would function with both instant-start and rapid-start ballasts. Preferably, such a lamp would also be effective to maintain the pressure of mercury vapor in the optimum range of 6-10 xcexcm when used in high-temperature enclosed lamp fixtures.
A low pressure mercury vapor discharge lamp is provided having a light-transmissive glass envelope having an inner diameter and an inner surface, with first and second lamp bases attached at each end of the glass envelope, a phosphor layer coated adjacent the inner surface of the glass envelope, a discharge-sustaining fill gas of mercury vapor and an inert gas sealed inside the envelope, first and second electrodes mounted within the envelope, and a mercury cold spot region at an end of the envelope behind the first electrode. Each electrode has has a mount height measured from the electrode to the proximate inner end surface of the glass envelope. The mount height of the first electrode is at least 31 mm.